From Curiosity, Another Martian Landscape

This image of the crater wall is north of the landing site, or behind the rover. Here, a network of valleys believed to have formed by water erosion enters Gale Crater from the outside. This is the first view scientists have had of a fluvial system - one relating to a river or stream — from the surface of Mars.

With the loose debris blasted away by the rockets, details of the underlying materials are clearly seen. Shown in the inset in the figure are pebbles up to 1.25 inches across (upper two arrows) and a larger clast 4 inches long protruding up by about 2 inches from the layer in which it is embedded.